This invention relates generally to a printing machine, and more particularly, concerns an improved copy sheet feeder for such a machine.
High speed xerographic reproduction machines and printers, such as, the Xerox DocuTech.RTM. 135 and Xerox.RTM. 5090 produce copies at a rate in excess of several thousand copies per hour, and therefore, the need for reliable high speed feeding of copy sheets is essential. Presently, some copiers and printers use top vacuum corrugation feeders with a front air knife. In this system, a vacuum plenum with a plurality of friction belts are arranged to run over the vacuum plenum is placed at the top of a stack of sheets in a supply tray. The vacuum system is sized such that there is high open port flow to be able to acquire sheets, but a lower closed port pressure as to not damage or smear the sheets. At the front of the stack, an air knife is used to inject air into the stack to raise the top several sheets from the remainder of the stack. The air pressure actually required to physically separate sheets 1 and 2 from the stack can vary greatly dependent on the basis weight, static conditions, curl conditions, and edge welding properties of the paper. The air knife, however, is designed based on a single air pressure setting for the air knife assembly. This air pressure must be adequate for basis weights from 56 gsm to 200 gsm. This is usually a mutually exclusive event. Therefore, a basic latitude issue arises as to the air pressure requirements for heavy versus lightweight paper. In addition, sheets which are curled in the upward direction, or a stack of sheets with edge welds, present acquisition difficulties. As a result, either the latitude is limited, or a much longer acquisition time is required. A long acquisition time implies that the feed rate is limited. Increasing the vacuum level means that the feeder may cost more and may be noisy. In operation, air is injected by the air knife toward the stack to separate the top sheet, the vacuum pulls the separated sheet up and acquires it. Following acquisition, the belt transport drives the sheet forward off the stack of sheets. In this configuration, separation of the next sheet cannot take place until the top sheet has cleared the stack. In this type of feeding system every operation takes place in succession or serially, and therefore, the feeding of subsequent sheets cannot be started until the feeding of the previous sheet has been completed. In addition, in this type of system, the air knife may cause the second sheet to vibrate independent of the rest of the stack in a manner referred to as "flutter". When the second sheet is in this situation, if it touches the top sheet, it may tend to creep forward slightly with the top sheet. The air knife then may force the second sheet against the first sheet causing a shingle or double feeding of sheets.
Also, some current top and bottom vacuum corrugation feeders utilize a valved vacuum feedhead, e.g., U.S. Pat. No. 4,269,406 which is included herein by reference. At the appropriate time during the feed cycle, the valve is actuated establishing a flow and hence a negative pressure field over the stack top or bottom if a bottom vacuum corrugation feeder is employed. This field causes the movement of the top sheet(s) to the vacuum feedhead where the sheet is then transported to the takeaway rolls. Once the sheet lead edge is under control of the takeaway rolls, the vacuum is shut off. The trail edge of this sheet exiting the feedhead area is the criteria for again activating the vacuum valve for the next feed. While these feeders are successful to some extent in feeding copy sheets at high rates of speed, there is still a need for a more reliable high speed feeder that is lower in cost, lower in noise level, and with increased feeder latitude and reduced shutdown rate than has been practiced heretofore.